It is now commonplace to explore for oil, gas and other valuable minerals by seismic exploration, in which seismic energy is imparted to the earth. The seismic energy passes through the earth, and is reflected at interfaces between rock layers of varying acoustic impedance. Its return to the surface of the earth can be detected by acoustic microphones referred to as geophones when used in land-based exploration, or as hydrophones when exploring the seabed. For the purposes of this invention, geophones and hydrophones are equivalent. The geophone output when plotted as a function of time is referred to as a trace. If the traces from a plurality of such geophones are plotted next to one another the wavelets caused by reflection of the seismic energy from a single subterranean layer line up, and a picture approximating the subterranean structure of the earth emerges which can be used by geophysicists in the search for oil, gas and other minerals.
It is well known that as the geophones become displaced further and further from the source of seismic energy, the relative amplitude of the received signal drops off due to divergence of the generally spherical wave front within the earth as it travels therethrough. It is known to apply automatic gain control to the fainter portions of a given trace so that the average amplitude of the trace is more close to constant, so as to enable more efficient analysis. However, insofar as the present inventor is aware, no method is available for calculating the amount of gain to be applied which is completely appropriate. In particular, no method is known which directly compensates for losses due to spherical divergence as well as for other signal losses due to reflectivity and the like.